1. Field of the Invention
The present invention relates to a solid-state image pickup device, a control method thereof and a camera.
2. Related Background Art
In U.S. Pat. No. 6,307,195, there is described a solid-state image pickup device in which charge spilling from a photodiode during accumulation of photoinduced charge flows into a floating diffusion (FD) via a transfer gate (TG). By reading out the charge flowing into the FD, dynamic range is extended.
The capacity of FD, contributing to the amplification factor of an amplifying unit, cannot have a large value. Accordingly, only a small amount of charge spilling from the photodiode (PD) unit functioning as a photoelectric conversion unit can be accumulated. Also, the PD is connected to the input unit of the amplifying unit within a pixel, so the FD cannot have a perfectly buried structure. Here, “buried structure” means a structure having a reverse conducting-type impurity diffusion region formed on the surface of an impurity diffusion region for forming an FD. With such structure, the generation of dark current on the surface of the impurity diffusion region for forming an FD is reduced. Consequently, the dark current in the impurity region for forming an FD is larger than that in the photoelectric conversion unit. Accordingly, the FD is not suitable for a node holding data. Charge spilling into the FD is lost as time passes, so a high S/N ratio cannot be expected for a signal from the high brightness portion.
In Japanese Patent Application Laid-Open No. 2004-335803, there is described a MOS solid-state image pickup device in which a plurality of light receiving units are arranged in an array on the surface of a semiconductor substrate and a signal at each light receiving unit is read out on a per light receiving unit basis, each said light receiving unit being provided with a first signal charge detecting unit for detecting a signal dependent on the amount of incident light, and a second signal charge detecting unit for capturing a part of excess carrier of the first signal charge detecting unit when the detection signal at the first signal detecting unit saturates and thereby detecting a signal dependent on the amount of captured charge.
According to Japanese Patent Application Laid-Open No. 2004-335803, as shown in the accompanying FIG. 2, there is provided the second signal charge detecting unit 38 which detects a part of saturating electron when the saturation of electron generated in the first signal charge detecting unit 31 occurs. Part of excessive carrier is captured and the rest is drained out to a vertical type overflow drain (OFD). The first and second signal detection units are separately provided.
However, to dump one part to the vertical type OFD and collect the rest into the second signal charge detecting unit 38, there is needed a technique for fabricating with considerably high accuracy both the potentials of a barrier unit 33 and the vertical type OFD. When this accuracy is not sufficient, a variation in the flowing-in amount between the vertical type OFD and the detection unit occurs. Accordingly, the flowing-in ratio varies according to sample, thus significantly reducing mass productivity.
Also, the charge dumping side has a structure of vertical type OFD, and the charge capturing side has a structure of horizontal type OFD. Thus barriers having a different structure must be exceeded in each side. More specifically, the spilling ratio is controlled by potential barriers based on density profiles determined by a different semiconductor process. In addition, the amount of spilling-out current typically varies exponentially relative to potential controlling voltage. Consequently, it may be difficult to control the spilling ratio finely and highly accurately. From the above described reason, there is also a drawback that the flowing-out ratio between the vertical type OFD and horizontal type OFD has a temperature dependence.
Furthermore, the vertical type OFD determines the saturation charge of PD. Accordingly, the range where the potential can be controlled is considerably limited. Practically, to compensate for this, it is essential for the horizontal type OFD structure to have a potential controlling mechanism. This causes problems of increased false signal (dark current) at a dark state, reduced yield associated with an increasing number of control lines, and the like.
An object of the present invention is to cause a part of the charge spilling from a photoelectric conversion unit to flow into a charge holding unit and thereby extend dynamic range and at the same time improve image quality.